Digital data transmissions typically provide data in bit serial fashion. The data transmission may be divided into data frames or words with preambles or flags identifying certain data segments. To accurately detect such frames, words and preambles, or obtain any information from the data transmission, however, a receiver must first establish bit level synchronization. The remote receiver must determine precisely when the transmission should be sampled to obtain a sequence of valid data bit values. Valid bit sample points are separated by known time intervals so the task of bit synchronization requires identification of one at least valid bit sample point as a time reference, with subsequent samples taken according to the known separation between bit sampling points. If an incorrect bit sampling point is identified, subsequent sampling of the transmission yields data more susceptible to noise and therefore less likely to be valid.
In some transmission protocols a separate bit sample clock establishes a time reference identifying bit sampling points. According to such transmission protocols, a receiver must monitor both a data signal and a time reference signal for use as a bit sample clock. Bit synchronization is achieved by sampling the data signal in response to the bit sample clock. This method requires tight synchronization between the bit sample clock and the data signal. If the bit sample clock varies relative to the data signal, then information extracted from the data signal is more likely to be invalid.
If a separate bit sample clock is not available, other methods of bit synchronization can be used. A receiver can monitor the data signal and, based on detected characteristics of the data signal, generate an internal bit sample clock. For example, by identifying state transitions in the data signal it is possible to derive an internal bit sample clock as a time reference. Such a method requires, however, clearly identifiable signal transitions for accurate time reference. Without readily identifiable signal transitions, additional signal processing steps may be required to reconstruct such transitions. Unfortunately, signal noise often affects the determination of bit sample points based on measurement of the data signal and suitable data sample points are not obtained.
Rapid bit synchronization may be required in some transmission protocols. For example, in a time multiplexed system where a battery powered remote receiver activates to monitor a data signal during an assigned time slot, rapid bit synchronization conserves battery power because overall receiver activation time is reduced. Generally, however, bit synchronization by reference to the data signal alone, i.e. without an external bit sample clock, is more accurately achieved over a relatively long sampling period, and remains subject to error due to signal noise.
Accordingly, it would be desirable to provide a bit synchronization method less affected by signal noise to avoid invalid data recovery, and more rapidly established to conserve battery power.